The present invention relates to a construction of an air cool turbine blade more particularly for use in the high-temperature stage of a gas turbine.
It has been well known in the art that maintaining high gas temperatures at the turbine inlet is one of the ways of reducing the specific fuel consumption and increasing the specific output of the gas turbine. To this end, the gas having extremely high temperatures in excess of allowable or tolerable temperature limits of the components of turbine blades is made to flow into the turbine inlet so that the turbine blades must be cooled.
Cooling methods which are very effective for cooling turbine blades in practice includes so-called convection cooling wherein cooling air from a compressor outlet is made to flow along the interior wall surfaces of a hollow turbine blade; so-called impingement cooling wherein jets of cooling air are impinged against the interior wall surfaces and socalled film cooling wherein cooling air is made to issue from the interior of the turbine blade and to flow along the blade surfaces to form films of cooling air. It is of course preferable to combine various cooling methods rather than to employ a single cooling system.
According to one prior art turbine blade cooling system, an insert formed with a large number of impingement holes is inserted in a hollow blade and is spaced apart therefrom a suitable distance so that a space of a suitable volume may be defined therebetween. Cooling air from a compressor outlet is introduced into the space within the insert and issues through the impingement holes to impinge against the interior wall surfaces in the space, thereby attaining impingement cooling. Thereafter cooling air is made to flow through this space so that convection cooling of the interior wall surfaces of the blade may be attained, and then cooling air is made to issue through ejection holes or slots formed through the wall of the blade to flow along the exterior surfaces, thereby forming films of cooling air and consequently attaining film cooling.
This arrangement utilizes the air passage defined between the insert and the blade in order to attain impingement, convection and film cooling. However, the temperature of cooling air rises after impingement and convection cooling so that satisfactory film cooling effects may not be attained. In some cases, there is only a small pressure difference available between the leading edge of the blade and a cooling air supply source. When such a small pressure difference is distributed for issuing jets of cooling air for impingement cooling, for causing convection cooling and for issuing cooling air for film cooling, the pressure differences assigned for impingement, convection and film cooling becomes further smaller so that neither satisfactory impingement, nor convection, nor film cooling may be attained.